The present invention relates generally to magnetron sputtering cathodes, and more particularly to a magnet housing for a magnetron sputtering cathode.
DC reactive sputtering is a most frequently used deposition process for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazing. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. This system may be referred to as a continuous in-line system or a glass coater.
Inside the vacuum chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about 3 millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. A layer of material to be sputtered covers the cathode surface. This surface layer is known as the target or the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the sputtering gas discharge to form a thin film, It is advantageous to produce the gas discharge in the presence of a magnetic field using an apparatus known as a magnetron with an array of magnets mounted in a fixed position behind the target and the magnetic field causing electrons from the discharge to be trapped in the field and travel in a spiral pattern, thereby creating a more intense ionization and higher sputtering rates.
The rotary or rotating magnetron, developed to overcome some of the problems inherent in the so-called planar magnetron, uses a substantially cylindrical cathode and target. The target is rotated continually over a magnet array which defines a sputtering zone, such that a new portion of the target is continually presented to the sputtering zone. This has the favorable effects of easing cooling problems and allowing higher operating powers. The rotation of the target also ensures that the erosion zone comprises the entire circumference of the cathode covered by the sputtering zone. This increases target utilization. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetron requires bearings to permit rotation, and vacuum seals for the drive shaft, the electrical conduits and the cooling conduits. Vacuum and rotary water seals have been used to seal around the drive shaft and the conduits which extend between the coating chamber and the ambient environment. Various mounting, sealing and driving arrangements are described in U.S. Pat. Nos. 4,443,318; 4,445,997; and 4,466,877, the entire disclosures of which are also hereby incorporated by reference.. These patents describe rotating magnetrons mounted horizontally in a coating chamber and supported at both ends, each end of the cathode being attached to a spindle.
It is often preferable, however, to support the magnetron at only one end designated as the drive end by a cantilever mount. The other end of the cathode may be referred to as the free end. Several examples of cantilever mounted rotary magnetrons are given in Design Advances and Applications of the Rotatable Magnetron, Proceedings of the 32nd National Symposium of the American Vacuum Society, Vol. 4, No. 3, Part 1, pages 388-392 (1986), the entire text of which is hereby incorporated by reference. A cantilever mounted magnetron usually includes a bearing housing containing a drive shaft, a rotary vacuum seal, and at least two bearings spaced along the drive shaft, one of which may function as a shaft seal. Cantilever-mounted magnetron removal does not require removal of one of the mounting structures to provide the necessary clearance, and cantilever-mounted magnetrons require only one rather than two rotary seals.
A rotating magnetron incorporating a cantilever mounted removable cathode detachable from its bearing assembly is described in U.S. Pat. No. 5,100,527, assigned to the assignee of the subject application, the entire disclosure of which is hereby incorporated by reference. Such a system allows, among other things, the cathode to be removed easily and without special equipment, thus reducing system down time both by reducing the time required to replace a cathode and by making simultaneous removal of two or more cathodes more practical. A cantilever-mounted cathode having low vacuum seal loads is described in U.S. Pat. No. 5,200,049, assigned to the assignee of the subject application, the entire disclosure of which is also hereby incorporated by reference.
The cantilever-mounted cathodes are cooled by a cooling fluid such as water which is introduced and removed at the drive end of the cathode. The cathode may be hollow and enclose a concentric cooling fluid tube having an end connected, at the cathode drive end, to a cooling fluid supply tube concentric with, and inside, a cathode drive shaft. At or near the other end of the cooling fluid tube, located inside the cathode near the free end, the cooling fluid is released inside the cathode. The cooling fluid then flows on the outside of the cooling fluid tube towards the drive end as it cools the cathode, and the cooling fluid is removed through the drive shaft. The fluid flow velocity towards the drive end may be relatively low, as it occurs through a space having a relatively large cross section. As a result, the cooling effectiveness is reduced. While the fluid flow velocity could be increased in principle by increasing the fluid flow rate, the flow rate is limited by the maximum pressure that can be provided by pumps and withstood by the sputtering equipment.
The cathode magnets may be suspended from the cooling fluid tube. During operation, the magnets are subject to a magnetic force oriented in the direction of the gas discharge. This force, whose strength has not been appreciated heretofore, tends to bend the magnet assembly and the cooling fluid tube to which it is attached. Such bending may alter the magnetic field in undesirable ways and may lead to structural damage to the cathode. Another force acting upon the magnets and the cooling fluid tube is their weight.
Accordingly, an object of the present invention is to provide a magnetron cathode with high cooling fluid flow velocity without increased cooling fluid flow rate.
Another object of the present invention is to provide a magnetron cathode wherein magnet assembly deformation due to magnetic forces and weight is prevented.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.